TeaThe amount of oxygen in Earth’s atmosphere makes it a habitable planet.
Twenty one percent of the atmosphere is made up of this life-giving element. But in the deep past—as far back as the Neoarchean epoch, 2.8 to 2.5 billion years ago— this oxygen was almost absent,
So, how did Earth’s atmosphere become oxygenated?
our researchpublished in nature geologyadds an intriguing new possibility: that at least some of Earth’s early oxygen came from a tectonic source via movement and destruction of Earth’s crust.
the arcane earth
The Archean era represents one-third of our planet’s history, from 2.5 billion years ago to four billion years ago.
This alien earth was a water-world, covered in green oceanimmersed in a methane haze and completely lacks multicellular life. Another exotic aspect of this world was the nature of its tectonic activity.
On modern Earth, the dominant tectonic activity is called plate tectonics, where oceanic crust – Earth’s outermost layer beneath the oceans – sinks into Earth’s mantle (the area between Earth’s crust and its core) at a point of convergence Which is called subduction zone. , However, there is considerable debate as to whether plate tectonics operated back in the Archean era.
A feature of modern subduction zones is their association oxidized magmas, These magmas form when oxidized sediments and bottom water – the cold, dense water near the ocean floor – introduced into the Earth’s mantle, It produces magma with high oxygen and water content.
Our research aimed to test whether the absence of oxidized material in Archean floor waters and sediments could prevent the formation of oxidized magmas. The detection of such magma in Neoarchean magmatic rocks may provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.
Experiment
We collected samples of 2750- to 2670-million-year-old granite rocks from the Abitibi-Wawa sub-province of the Superior Province—the largest preserved Archaean continent stretching 2000 km from Winnipeg, Manitoba to far-eastern Quebec. This allowed us to probe the level of oxidation of the magma generated in the Neoarchean era.
It is challenging to measure the oxidation-state of these magmatic rocks that form through the cooling and crystallization of magma or lava. Post-crystallization events may have modified these rocks through subsequent deformation, burial, or heating.
So we decided to look minerals Appetite which is present in zircon crystal in these rocks. Zircon crystals can withstand the intense temperatures and pressures of post-crystallization events. They retain clues about the environments in which they were originally formed and provide precise ages for the rocks themselves.
Tiny apatite crystals that are less than 30 microns wide – the size of a human skin cell – are trapped in the zircon crystals. They contain sulphur. By measuring the amount of sulfur in apatite, we can establish whether apatite evolved from oxidized magma.
we were able to successfully measure oxygen intensity of the original Archaean magma – which is essentially the amount of free oxygen in it – using a special technique called X-ray absorption near edge structure spectroscopy (XSAS).S-XANES) at the Advanced Photon Source Synchrotron Argonne National Laboratory in Illinois,
Making oxygen from water?
We found that the magma sulfur content, which was initially around zero, increased to 2000 parts per million in about 2705 million years. This indicated that the magma had become more sulphurous. apart from this, Predominance of S6+ in apatite – a type of sulfur ion suggested that sulfur corresponds to an oxidized source Data from the host zircon crystal.
These new findings indicate that the oxidized magmas formed in the Neoarchean era, 2.7 billion years ago. The data suggest that the lack of dissolved oxygen in Archaean ocean reservoirs did not prevent the formation of sulphur-rich, oxidized magma in subduction zones. The oxygen in these magmas must have come from some other source, and was eventually released into the atmosphere during volcanic eruptions.
We found that the occurrence of these oxidized magmas is related to major gold mineralization events in the Superior Province and Yilgaran Craton (Western Australia), demonstrating a connection between these oxygenated sources and global world-scale ore deposit formation.
While the exact mechanism is unclear, the occurrence of these magmas suggests that the process of subduction, where ocean water is carried hundreds of kilometers into our planet, generates free oxygen. It then oxidizes the upper mantle.
Our study suggests that Archaean subduction may have been an important, unexpected factor in the oxidation of Earth, the early Breath of oxygen 2.7 billion years ago and also The Great Oxidation Event, which marked a two percent increase in atmospheric oxygen from 2.45 to 2.32 billion years ago,
As far as we know, Earth is the only place in the Solar System – past or present – with plate tectonics and active subduction. This suggests that this study may partially explain the lack of oxygen and eventually life on other rocky planets in the future.
david mollPostdoctoral Fellow, Earth Sciences, Laurentian University, Adam Charles SimonArthur F. Thurnau Professor, Earth and Environmental Sciences, University of MichiganAnd zhuang mengPostdoctoral Fellow, Earth and Environmental Sciences, University of Michigan
This article is republished from Conversation Under Creative Commons Licence. read the original article,
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